1. Field of the Invention
This invention relates to manufacturing of integrated circuits (IC), and in particular to the reduction of metal oxides on a substrate into elemental metals during the manufacturing of ICs.
2. Description of the Related Art
Interconnects, capacitor electrodes and gate metals in integrated circuits (IC) are made of metals. Especially copper is an attractive alternative for the interconnects due to its favourable electrical properties, such as low resistivity.
During the processing of integrated circuit a metal surface may become oxidised, whereby the resistivity of the metal surface and the interconnect thereof increases. An oxide layer on a metallic interconnect restricts the flow of electrons through the interconnect and is detrimental in high-speed applications of ICs.
On the other hand, insulators are needed in the integrated circuits to separate the metal wires electrically from each other. Typically the low dielectric constant materials that can be used as insulators do not endure high temperatures, i.e. temperatures of approximately 400° C. or more.
So called damascene and dual damascene structures (c.f. FIG. 8) are common thin film structures applied in the manufacturing of ICs. During the manufacturing of damascene and dual damascene structures, openings (vias) are made through an insulating layer to an underlying copper metal layer. Thus, insulator surface is exposed to gas atmosphere. It is detrimental to the electrical properties of insulators to evaporate any volatile metal compound from the bottom of the openings and chemisorb even one molecular layer of copper compound on the insulator surface. Damascene processing requires a diffusion barrier on the insulator surfaces to prevent the diffusion of copper to the insulator. After a diffusion barrier layer is deposited, it is not possible to remove an oxide layer from the copper surface located on the bottom of the vias. It is obvious that there is a need for method where metal oxide can be removed without forming any volatile metal compounds.
According to prior art an oxide layer on a metal layer can be removed or reduced. U.S. Pat. No. 5,939,334 and EP publication No. 0 880 168 disclose a method of removing metal oxide layer by β-diketones. β-diketone vapour is contacted, e.g., with a copper oxide surface where copper β-diketonate and water are formed. Both reaction products are removed from the surface by evaporation and a metal surface is obtained. The process of the publication can not be successfully applied to the production of integrated circuits, since reliability problems occur due to metal contamination of the solid material. Volatile metal compound molecules chemisorb from the vapour phase onto the substrate surfaces surrounding the conductors, from which surface they diffuse into the insulator.
According to the method disclosed in U.S. Pat. No. 6,033,584 H2 plasma treatment can be used for the reduction of copper oxide layer. Although the method can be used even at low temperatures, plasma tends to damage the insulator materials of an IC. Furthermore a plasma generator adds cost and complexity to the overall IC manufacturing process.
It is known that copper oxide can be reduced by hydrogen gas and carbon monoxide. A method of manufacturing semiconductor metal wiring layer by reduction of metal oxide by hydrogen of carbon monoxide has been described in U.S. Pat. No. 5,731,634. Non-activated hydrogen has a rather strong H—H bond which requires either increased process temperatures or additional energy, e.g. in the form of plasma.
M. Utriainen at al. studied the reduction of nickel oxide with hydrogen (Applied Surface Science 157 (2000) pp. 151-158). Nickel may catalyse the breakage of the H—H bond and thus activate hydrogen gas. According to the publication the reduction of NiO could be done at 230° C. in 30 minutes. However, the authors state that the mentioned reduction step induced structural collapse of the thin film and pinholes were formed.
Small molecules such as H2 and CO quickly diffuse inside a metal oxide film and form by-products (H2O and CO2) inside the film. Due to increased size, these by-products have lower diffusivity towards the surface than the reducing H2 or CO molecules towards the film. Increasing internal pressure may rupture the film and create pinholes. Therefore, the use of H2 or CO as reducing agents is not favourable in the applications of the present invention.
The inventor made comparative experiments with hydrogen and ammonia without a catalyst. Ammonia was selected for making experiments because it is rich in hydrogen. The result was that hydrogen did not reduce copper oxide at 450° C. in one hour (Finnish patent application no. FI20001163). The inventor obtained the same result with gaseous ammonia: copper oxide could not be reduced at a temperature lower than 500° C. IC processing benefits from a simple reduction process of the present invention that works below 400° C. quickly enough (<5 minutes) and does not contaminate insulator surfaces.
Chemical reactions between copper oxide(s) and alcohols, aldehydes and carboxylic acids are known in literature (Gmelin Handbuch der Anorganischen Chemie, Kupfer, Teil B-Lieferung 1, System-Nummer 60, Verlag Chemie, GMBH., 1958). The experiments described in the publication were, however, carried out with copper oxide powder which may have large surface-to-volume ratio due to rough surface and thus is more easily reduced to copper metal than copper oxide in the form of a dense smooth layer is. No reaction conditions were given. Moreover, the publication does not discuss the possibility of using the reduction reaction for making metal thin films or in the process for producing integrated circuits.
EP 0 469 470 A1 discloses a process for reducing copper oxide on printed board. The application concerns copper foil conductors on a printed circuit board. Hydrogen, carbon monoxide or mixtures thereof or hydrazine were used as reducing agents. Platinum metal group metal catalyst is fixed on a copper oxide layer. The catalyst activates the reducing agents at low temperatures. Copper oxide was used to improve the adhesion between copper metal and an insulating laminating (matrix resins) layer. Copper oxide was removed from the copper surface by reduction.
The disadvantage of the method of reducing copper given in EP 0 469 470 A1 is that an activator metal (platinum group) is deposited on the surfaces. The platinum group metals include ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt). Depositing a uniform platinum group metal coating on the vias and trenches of damascene structures is a complicated and costly process with PVD and CVD methods.
EP 0 469 470 A1 mentions a generation of reducing gas (hydrogen or carbon monoxide) through catalytic pyrolysis of alcohols, aldehydes, carboxylic acids, ammonia and hydrazine. The reducing gas is then contacted with the metal oxide. A disadvantage of the method is that catalytic pyrolysis may form non-volatile carbon-rich fragments from organic compounds that contaminate surfaces. These non-volatile fragments may not harm the functionality of macroscopic contacts (diameter about 0.1-1 mm) in printed circuit boards but these fractions are detrimental to microscopic contacts (diameter about 0.18-0.25 μm) in integrated circuits.
EP 0 469 456 A1 discloses a method of removing copper oxide from copper clad laminates used in printed circuit boards. Reducing gases are generated via catalytic pyrolysis of hydrazine and methylhydrazine. Hydrazine and methylhydrazine are harmful chemicals with carcinogenic and corrosive nature and pyrolysis may generate condensable compounds from methylhydrazine that contaminate sensitive surfaces of ICs. The contact areas made to the copper surface of a printed circuit board are of macroscopic scale (diameter in the order of 0.1-1 mm). The publication does not teach how to apply the invention to the manufacturing of microscopic damascene structures.
A method of liquid-phase removal of copper oxide from copper surfaces is known from WO 93/10652 A1 and DE 41 08 073. The disadvantage of a liquid-phase based method is that metal oxide is reacted into the form of a soluble metal compound that can adhere as a molecular layer to all surfaces exposed to the solution. Furthermore, it is costly to apply the liquid phase removal of metal oxide to a damascene process. For example, after liquid phase removal of copper oxide the substrate must carefully be protected against any exposure to atmosphere that contains oxygen or reactive oxygen compounds before the deposition of a diffusion barrier. Many damascene process steps are preferably done in a cluster tool that only consists of vapour-phase processing units. The copper metal surface provided during the liquid-phase treatment is easily re-oxidised into copper oxide before the substrate arrives in a vapor-phase processing unit. Combining liquid-phase and gas-phase units is complicated and costly. Furthermore, it is difficult to apply the liquid phase treatment to the removal of metal oxide from the via bottoms in damascene and dual damascene structures since the IC substrate should be rinsed and dried carefully in oxygen-free atmosphere and transported in perfectly oxygen-free atmosphere or high vacuum environment to the cluster tool to avoid the re-oxidation of the copper surface.